The present invention relates to a coated cemented carbide insert particularly useful for dry milling of extremely highly alloyed grey cast iron in particular, grey cast iron further alloyed with carbide forming elements such as Cr, Ti and Mo.
U.S. Pat. No. 6,177,178 discloses a coated milling insert particularly useful for milling in low and medium alloyed steels with or without raw surface zones during wet or dry conditions. The insert is characterized by a WC—Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an inner layer of TiCxNyOz with columnar grains, a layer of κ-Al2O3 and, preferably, a top layer of TiN.
U.S. Pat. No. 6,333,098 discloses a coated cutting insert particularly useful for dry milling of grey cast iron. The insert is characterized by a WC—Co cemented carbide substrate and a coating including an innermost layer of TiCxNyOz with columnar grains and a top coating of a fine-grained α-Al2O3 layer.
Grey cast-iron is a material, which, in general, is reasonably easy to machine with cemented carbide tools. Often, long tool life can be obtained. However, the machinability of cast iron can vary considerably. The tool life may be influenced significantly by small variations in the chemical composition within the material, which may be related to the casting technique used, such as the cooling conditions. Other causes for variations are the casting skin and sand inclusions, if present, or even the stability of the machine used for cutting the material.
Extremely highly alloyed cast iron is used in some types of car engines in order to obtain high strength of the engine.
With extremely highly alloyed grey cast iron is herein meant cast iron alloyed with additives such as Cr, Mo, Ti, which form different types of carbides in the matrix. Preferably, the contents of these elements are Cr greater than about 0.15, Mo greater than about 0.04, and/or Ti greater than about 0.015 weight-%.
When machining such cast irons by coated milling inserts, abrasive wear type is dominating because of the presence of the carbides. That is, fragments or individual grains of the layers and later also parts of the cemented carbide are successively pulled away from the cutting edge by the work piece chip formed. Such wear mechanism is further accelerated by the formation of cracks about 50 μm apart along and perpendicular to the cutting edge. As soon as the coating is worn out, the underlying cemented carbide will wear fast. The increased specific surface contributes to higher cutting forces, increased temperature and increased chemical reaction between workpiece material and the coating/substrate.